hi all....
i want to design a MOSFET amplifier to drive 4 or 8 ohm speakers, but i cant find MOSFET biasing equations .
i would appreciate if someone explained them to me or pointed me to a website where i can find them .

thank you for your help.
i couldn't clearly understand this article and how to choose resistors values and it looks complicated .am just a beginner ,is there any other simpler circuits like this one , but with smaller supply voltage.

You may want to start off with the e-book at the top of every page on this site.

Your voltage is not high for an audio amp. And you will not get "high" volume levels with lower voltages. If you are a beginner, you will want to read through the basics of op-amps and audio amplification in addition to basic electronics and safety. You will be dealing with capacitors that can bite you. And some can bite you REAL hard.

You can use 5 9v batteries as a power source for the circuit. In parallel, that will give you 45v. Use a voltage regulator to 40v. Depending on your amp draw, you can even step one 9v battery to 40v. Tasers step one 9v battery to 10,000v at extremely low amperages.